While optical detectors and imaging devices may be configured to operate within widely varying wavelengths of interest, the discussion that follows will focus upon Mid-Wave Infrared (MWIR) and Long-Wave Infrared (LWIR) spectral bands.
Bolometers transduce an increase in temperature into a change in resistivity. Materials with high temperature coefficients of resistivity such as amorphous silicon and vanadium dioxide may be used as the detector elements in microbolometers.
Available variants for high optical absorption may include applied coatings of infrared (IR) absorbing material having high intrinsic loss, resonance cavities, and metamaterial or surface plasmon based absorbers. There are several materials with intrinsic loss that strongly absorb electromagnetic radiation over large band-widths such as graphite, metal-black, and carbon nanotubes. However, thicknesses in the 10-100 μm range are typically required for those materials to achieve strong absorption in the longer IR regions. Such thick films have disadvantages to include difficulty in patterning, low thermal and mechanical stability, and reduction of absorption over time. Also, these materials are not design tunable.
The Fabry-Perot type of cavity is popular in thermal detectors. However, absorption bands from Fabry-Perot type of resonance cavities are highly angle dependent and the quality factor of such resonators can be difficult to control in manufacturing. Metamaterial or surface plasmon based absorbers, with the names being exchanged in the literature rather loosely, usually consist of thin layers of metal-dielectric-metal and have been reported to produce polarization insensitive strong absorption over wide angles. Typically, such absorbers consist of a periodic metallic squares, a dielectric spacer layer and a metal ground plane, that is, it has a metal-dielectric-metal (MDM) structure. The resonance frequency band is only several hundred nanometers wide, depending on the size and periodicity of the surface structures, and thickness and refractive index of the dielectric.
Current infrared imaging array detectors employ relatively crude wavelength selection techniques by being designed to match broad atmospheric transparency windows. These are the MWIR and LWIR bands, corresponding to 3-5 and 7-12 μm wavelengths, respectively. The MWIR band is of interest for imaging relatively hot targets, such as engines and rocket plumes. The LWIR band is valuable for night-vision and imaging of targets such as humans, animals, and other structures having a temperature that is slightly elevated above ambient air temperature.
Common existing-technology uncooled LWIR detectors include a VOx microbolometer with 17 μm pitch, 30 mK NETD (noise-equivalent temperature difference), and a 10 ms response time. The D* value is well into the 109 cm√Hz/W range. Premium commercially-available detectors comprise a 12 μm pitch with a 40 mK NETD and 10 ms response time. Inexpensive (˜$250) low-resolution, wafer-level-packaged FPAs are available as commercial off the shelf solutions.
At the other end of the performance spectrum, expensive high-definition cameras are available. Their costs are mainly driven by the large and fast optics rather than by the FPA (focal plane array). Achieved absorptance is near unity for such microbolometers across the LWIR band, and speed-responsivity trade-off is nearly optimized. Over the next few years, further pitch reduction is expected, to 10 or 12 μm, and NETD may be reduced to 10 mK using doped-polycrystalline VO2. This will likely result in detectors that are nearly thermal-fluctuation-noise limited, as opposed to the present state of the art wherein Johnson noise dominates.
At that point, the detectors will have reached fundamental theoretical limits for microbolometers (D*˜109-1010 cm√Hz/W), and additional research and development dedicated to improving standard figures of merit are expected to yield strongly diminished returns.
Microbolometers, in their current form, have sufficient sensitivity for general imaging applications. However, they cannot be used for spectral sensing, where narrow-band wavelength selectivity is integrated directly into the detector. This yields a non-trivial limitation when applied to certain target-identification tasks. For example, it is possible to distinguish targets based on subtle differences in its emissivity spectrum, which can arise due to the effects of sharp molecular absorption features. It may be possible to remotely ascertain the identity of certain factory waste gases by analyzing the spectral emission therefrom. To perform such spectral discrimination now, using current-technology bolometers, would require considerable fore-optics and spectroscopic instrumentation, such as gratings or interferometers.
Accordingly, there is an unmet need in the art for improved wavelength-selective MWIR and LWIR detectors and related methods.